1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by using an electrophotographic system, and more particularly, it relates to an image forming apparatus such as a copying machine, a printer and the like, including an electrophotographic photosensitive member as an image bearing member and wherein an image is written on the image bearing member by charging and exposing and the image is formed on a transferring material by an image forming process including a developing step, a transferring step using transferring voltage and the like.
2. Related Background Art
In the past, as full color image forming apparatuses using an electrophotographic system, apparatuses of the type adopting an intermediate transferring system have been used more and more.
The intermediate transferring system is a system in which a developer image (toner image) formed on an image bearing member (photosensitive member) is primarily transferred on an intermediate transferring member provided as a second image bearing member in such a manner that, in particular, a plurality of color toner images are successively superimposed and thereafter the toner images laminated or superimposed by the primary transferring are secondarily transferring collectively on a transferring material such as a paper (recording material) or the like.
In this system, in comparison with a direct transferring system in which the toner images from the photosensitive member are superimposed directly on the transferring member, i.e., in comparison with an image forming apparatus in which, for example, the transferring material is conveyed to a contact position between the transferring material and the photosensitive member while fixing or adhering the transferring material to a transferring material conveying member for conveying the transferring material to the contact position and the toner images are transferred from the photosensitive member, since the toner images can be transferred from the intermediate transferring member without any treatment and control on the transferring material (for example, the transferring material is gripped by a gripper or the transferring material is absorbed or the transferring material is curved with a certain curvature or the like), there is obtained an advantage that various kinds of transferring materials from a thin paper (40 g/m2 paper) to a thick paper such as an envelope, a post card, a label paper and the like and transferring materials having various widths and lengths can be selected optionally.
Further, in comparison with a case where a rigid cylinder such as an intermediate transferring drum is used by designing the intermediate transferring member in a belt configuration, degrees of freedom of installation of the intermediate transferring member within the image forming apparatus are increased, so that there is obtained a merit that a main body of the apparatus can be minimized and a cost can be reduced due to effective use of a space.
Regarding the color image forming apparatuses using the plurality of color developers and adopting the intermediate transferring system, in particular the color image-forming apparatuses using the intermediate transferring belt, the following two main types are known:
One of such types is a first type (a so-called four-cycle type) apparatus in which a plurality of developing apparatuses containing different color developers are provided in an image forming portion including one photosensitive member and, when a plurality of color toner images corresponding to image information are formed on the rotating photosensitive member successively for each color by the electrophotographic process, when an every color toner image is formed, the image is primarily transferred onto the intermediate transferring belt contacted with the photosensitive member in the superimposed manner, and the superimposed toner images on the intermediate transferring belt are ultimately transferred onto the transferring material collectively. Namely, the image forming apparatus is designed so that only one image forming portion including one photosensitive member is provided and, following the formation of the toner image on the photosensitive member, the primary transferring onto the intermediate transferring belt is repeated a number of times corresponding to the number of colors to form the plural color composite toner images on the intermediate transferring belt and the toner images are secondarily transferred onto the transferring material collectively.
The other type is a second type (a so-called tandem type) apparatus in which a plurality of image forming portions each including one photosensitive member are provided and the toner image having a color corresponding to the image information is formed on each photosensitive member of each image forming portion by the electrophotographic process and the so formed toner images are primarily transferred, in the superimposed manner, onto the intermediate transferring belt rotated while contacting with the photosensitive members of the plurality of image forming portions and the superimposed toner images on the intermediate transferring belt are ultimately transferred onto the transferring material collectively. Namely, the image forming apparatus is designed so that the photosensitive members are provided in the respective plurality of image forming portions, and, when the intermediate transferring belt is passed by the respective image forming portions, the plurality of toner images are primarily transferred to form the composite toner images and, thereafter, the toner images are secondarily transferred onto the transferring material collectively.
In the image forming apparatuses of the above-mentioned two types, regarding the transferring of the toner images from the photosensitive member(s) to the intermediate transferring belt, in many cases, such transferring is carried out by a transferring device in which a transferring member such as a primary transferring roller contacting with a back side of the intermediate transferring belt is provided in a contacting portion (transferring portion) between a photosensitive member and the intermediate transferring belt and the toner images on the photosensitive member(s) are electrostatically transferred onto the peripheral surface of the intermediate transferring belt by applying transferring voltage (transferring bias) having polarity opposite to charging polarity of the developer (toner) to the transferring member(s).
However, in such image forming apparatuses, there arose a problem that, immediately after the toner image is primarily transferred to the intermediate transferring belt, a part of the developer (toner) constituting the toner image, and particularly of the toner constituting corners of the toner image is dispersed around the toner image, thereby deteriorating image quality.
Such toner dispersal phenomenon is considered to be caused by the following reason.
In a reversal developing system, in comparison with an image area as an image part (exposure part) formed on the photosensitive member on which the toner is rested, a non-image part (non-exposure part) on which the toner is not rested has a high potential condition that potential of the photosensitive member is maintained to primarily charged charging potential and, since there is no toner layer, a thickness of a dielectric layer between the photosensitive member and the transferring member becomes small, so that electrostatic capacity of the non-image part becomes great. In this condition, if the process for transferring the toner image from the photosensitive member to the intermediate transferring belt is carried out, in the area of the non-image part, more charges than in the area of the image part are applied to the intermediate transferring belt. As a result, as shown in FIG. 9, regarding charge distribution on the intermediate transferring belt, plus charging density of the non-image part having no toner becomes greater than that of the image part including the toner.
Since the applied charge has polarity opposite to the charging polarity of the toner, due to such charge distribution, a force for dispersing the toner acts on the toner on the intermediate transferring belt, with the result that, the toner of the toner image is dispersed on a portion of the intermediate transferring belt which has past through the transferring portion.
Particularly, such toner dispersal phenomenon tends to occur immediately after the second color toner image and so on is transferred. Further, also in a case where the developer includes spherical toner, i.e. substantially spherical particles, such a phenomenon tends to occur.
In order to reduce the charge applied to the portion of the intermediate transferring belt corresponding to the non-image portion, Japanese Patent Application Laid-open No. H5-165383 describes a method (pre-transferring exposure) in which a whole longitudinal area of the photosensitive member is exposed before the transferring. In this case, however, a toner holding force on the photosensitive member is reduced, thereby causing a problem that the toner is dispersed before the transferring.
Further, Japanese Patent Application Laid-open No. 2002-72602 describes a technique in which toner having opposite polarity is adhered to a non-exposed portion in the vicinity of a boundary between an exposed portion and the non-exposed portion and, thereafter, light is illuminated onto the surface of the photosensitive member by an electricity removing lamp. With this arrangement, in the Japanese Patent Application Laid-open No. 2002-72602, the toner having the opposite polarity can prevent the exposure of the non-exposed portion in the vicinity of the boundary between the exposed portion and the non-exposed portion, with the result that the toner adhered to the exposed portion is held by the potential which is not completely reduced in the vicinity of the boundary. However, due to such potential which is not completely reduced in the vicinity of the boundary, much charge is applied to the non-image portion of the belt, with the result that the toner dispersal cannot be prevented properly in the transferring portion.
Incidentally, Japanese Patent Application Laid-open No. H11-24425 describes a technique in which volume specific resistance is specified and an intermediate transferring belt has an outermost layer including anti-weather adding agent, and, Japanese Patent Application Laid-open No. 2003-57963 describes a technique in which, after transferring voltage is applied by the transferring member, voltage having polarity opposite to that of the transferring voltage is applied from the back side of the intermediate transferring belt. However, in both cases, it is not possible to prevent the charge having the same polarity as that of the transferring bias from being applied to the area of the intermediate transferring belt corresponding to the non-image portion.